Expectant mothers living in large towns or cities are a third more likely to give birth prematurely because of pollution, research suggests.

Traffic fumes are the biggest culprit, with chemicals called polycyclic aromatic hydrocarbons (PAH), a by-product of petrol, producing a 30 percent increase in risk.

Exactly why these components of air pollution were linked to premature cardiac defects is unclear, but the fetal heart is still developing in the second month of pregnancy--when pollution exposure was key in this study. Other birth defects such as cleft palate were not related to a mother's exposure to air pollution.

Ammonium nitrate from agriculture and industry heightened the threat of premature birth by a fifth -- 21 percent -- while benzene, a petrochemical, and diesel fumes caused a 10 percent increase.

Researchers also noted the concentrations of pollutants were higher in winter than in summer, and coastal cities had cleaner air than those further inland.

"While we can't estimate the precise increase in cancer risk, these findings underscore the need for policymakers at the federal, state, and local levels to take appropriate steps to protect children from these avoidable exposures."

Study author Dr Beate Ritz said: ‘Air pollution is known to be associated with low birth weight and premature birth.

‘Our results show traffic-related PAH are of special concern as pollutants and that PAH sources besides traffic contributed to premature birth.

‘The increase in risk due to ammonium nitrate particles suggests secondary pollutants are also negatively impacting the health of unborn babies.’

Researchers from the University of California looked at 100,000 births within a five-mile radius of air quality monitoring stations in the state, where Los Angeles is notorious for car-related air pollution.

The results, published in the journal Environmental Health, revealed that an increased concentration of PAH, benzene or diesel could increase the risk of giving birth prematurely.

Dr Ritz said: ‘Some pollutants were area-specific, relating to industry and urbanisation. However, overall exposure to critical pollutants such as PAH resulted in up to a 30 percent increase in the risk of premature birth.

‘Other toxic substances, such as benzene and fine particulate matter from diesel fumes, were associated with a 10 percent increase, while ammonium nitrate fine particles were associated with a 21 percent increase.’